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30, June 1908 : The Tunguska Event

"It was nothing of this earth, but a piece of the great outside; and as such dowered with outside properties and obedient to outside laws.""The Colour Out of Space", by H.P. Lovecraft (1927)

In the morning of June, 30 1908 eyewitnesses reported a large fireball crossing the sky above the region of the Stony Tunguska (PodkamennayaTunguska) in Siberia. A series of thunder was heard to the village of Achajewskoje - in a distance of 1.200 kilometres.At the same day at various meteorological stations of Europe seismic and pressure waves were recorded, and in the following days strange atmospheric phenomena were observed, silvery glowing clouds, colourful sunsets and luminescence in the night.

Local newspapers in Russia reported about a meteorite impact based on the eyewitness reports, international newspapers speculated about a possible volcanic explosion - the events after the eruption of Krakatoa in 1883 were still vivid in memories.Dr. Arkady Voznesensky (1864-1936), Director of the Magnetographic and Meteorological Observatory at Irkutsk from 1895 to 1917, collected the eyewitness records and proposed as first the hypothesis of an extraterrestrial impact, however the inaccessibility of the area and the political situation in Russia prevented further research.Only 13 years later the Russian mineralogist Leonid Alexejewitsch Kulik, reading some of the eyewitnesses' accounts about an explosion and large glowing object above the Taiga, became interested in the phenomena - there was also the hope to recover precious extraterrestrial metals from the supposed meteorite.Kulik first had to travel to the city of Kansk, where he researched ulterior accounts or reports in the local archives. Most of the recuperated stories refer to large fireballs, flames and up to 14 subsequent thunders. In March 1927 he arrived at the outpost of Wanawara and then on 13, April Kulik discovered a large area of 2.150 square kilometres where all the trees were torn apart, lying on the ground.

Fig.1. The forest of Tunguska completely leveled by the shock wave of the explosion, photograph taken by Evgeny Krinov in 1929.

Despite his intensive survey, he didn't locate a single great impact crater as expected, but he found some circular pits that he interpreted as impact craters; however no meteoritic material was discovered in the entire area.In autumn 1927 a preliminary report by Kulik was published in various national and international newspapers, the destroyed forest and the event that he described became later known as "Tunguska Event".Kulik formulated one of the first hypotheses to explain the phenomena and the lack of evidence on the ground, he proposed that a bolid exploded already in the atmosphere, causing the explosion and devastation, single minor fragments became buried in the swampy ground, which was soft enough to collapse above them and didn't preserve the typical morphology of an impact crater.In 1929 Kulik returned to the supposed impact area for three times, excavating and drilling into the swamp, hoping to discover some fragments of the bolid, however unsuccessful.Kulik will die in 1942 in German imprisonment.

In 1934 Sowjet scientists proposed a new comet-hypothesis trying to explain the apparent lack of extraterrestrial material: a comet composed principally of ice entered the atmosphere, exploded and vaporized completely.

The lack of direct evidence generated many speculations and hypothesis:

Between 1945 and 1959 the engineer Aleksander Kasantsews formulated, based on the impression left by the first atomic bombs, an unusual explanation involving a nuclear explosion of possible extraterrestrial origin.In 1973 American physicists published in the journal Nature the idea that a small black hole collided with earth, causing some sort of matter-antimatter explosion.The German astrophysician Wolfgang Kundt and subsequent Jason Phipps Morgan of the Cornell University in Ithaca and Paola Vannucchi from the University of Florence proposed in the last years an ulterior hypothesis: "Verneshots", in reference to the author of the novel "A Journey to the Center of the Earth", are supercritical magma/gas mixtures erupting violently from the underground. According to the proposed model in areas with a thick earth crust or composed of resistant rock (the region of Tunguska is part of the basaltic Siberian Trapps) magmatic intrusions and gases tend to build up pressure until the rocks were shattered to pieces, hot gases can violently escape, ignite and causing an explosion.

However the most compelling theory remains the impact of a natural extraterrestrial object. This hypothesis is supported by the reports describing a fireball descending on the Tundra in 1908, sedimentary features (the supposed presence of nanodiamonds, magnetic spherules and silicate spherules in the sediments of the swamp) and the pattern of the tree logs, supporting an explosion in midair.There are some inconsistencies according to critics of the impact scenario: accounts of a series of thunders over a longer time period are hard to reconcile with a single impact (possibly various fragments ?) and the recovered sediments are not unambiguous (common background sedimentation or of not extraterrestrial origin ?) - the best evidence would be a fragment of the impactor.In 2007 Luca Gasperini and his research team of the University of Bologna proposed a small lake as possible impact crater of a fragment of the meteorite that caused the explosion, lake Cheko is unusually deep unlike other lakes in the region and was apparently not reported previously of 1908 (however the region was poorly mapped and explored at the time). Also here the proposed evidence was not undisputed as seen in the response by COLLINS et al. 2008.Only the discovery of material of extraterrestrial material on the bottom of the lake would be a decisive argument to settle the discussion of the mystery of Tunguska.